1. Field of the Invention
The present invention relates to a driving device, and particularly to a driving device which comprises an electromechanical conversion element such as a piezoelectric element or the like. The driving device of the present invention is suitably used as a lens-driving mechanism of a camera or a driving mechanism of a precision stage.
2. Description of the Related Art
The Japanese patent publication of JP-A-11-98865 discloses a driving device as shown in the exploded perspective view in FIG. 1A and the assembly diagram thereof in FIG. 1B. This driving device comprises an electromechanical conversion element such as a piezoelectric element, of which the length changes (expands or contracts) under application of a voltage.
In this driving device, the slider 10 can be moved relative to the base 1. For example, this driving device can be used as a lens-driving device for a camera. That is, a lens frame of the camera is connected with the slider 10, such that the associated lens is driven together with the slider 10.
The piezoelectric element 4 is composed of a plurality of laminated piezoelectric plates. One end 4a of the piezoelectric element 4 in its expanding or contracting direction is fixed to the base 1, and the other end 4b is fixed to one end 5a of the rod 5. The rod 5 is slidably supported by the support walls 2 and 3 which are integrally formed with the base 1.
The slider 10 is frictionally engaged with the outer surface of the rod 5, wherein the rod 5 is sandwiched between the body 11 and the cap 12 of the slider 10, and a leaf spring 13 imparts an urging force to the body 11 and the cap 12 in approaching direction.
The piezoelectric element 4 is connected to a voltage-controlling circuit or drive-pulse generator (not shown). When a predetermined drive voltage with sawtooth-like waveform is applied to the piezoelectric element 4, the piezoelectric element 4 vibrates while drawing substantially the same sawtooth-like displacement (FIG. 2). With this vibration, the rod 5 also vibrates in its longitudinal direction, while drawing sawtooth-like displacement. That is, the graph in FIG. 2 shows the displacement of the rod 5 due to the vibration, as well as the displacement of the piezoelectric element 4 due to the vibration.
This is described in more detail. During the interval “A” of the first waveform 100, a gentle rising inclination 101 is shown, and during this interval, the piezoelectric element 4 expands at a relatively slow rate, and the rod 5 slowly moves in the arrow direction I in FIG. 1B. Next, during the interval “B”, the piezoelectric element 4 abruptly contracts to its initial length (the portion of the waveform indicated by the falling inclination 102), and the rod 5 rapidly moves in the arrow direction II.
The same is repeated thereafter, and consequently, the rod 5 vibrates while alternately repeating the slow motion in the direction I and the rapid motion in the direction II. In this way, the rod 5 vibrates while drawing sawtooth-like vibration waveforms, with gentle and acute alternations.
The force of the leaf spring 13 of the slider 10 (i.e., the frictional engaging force of the slider 10 to the rod 5) is adjusted, such that when the rod 5 slowly moves (in other words, when the rod 5 moves at a rate smaller than a predetermined value), the slider 10 moves together with the rod 5, and when the rod 5 rapidly moves (in other words, when the rod 5 moves at a rate larger than the above predetermined value), the slider 10 stops there due to its inertia, or moves a little compared with that of the rod 5, as shown in FIG. 3. As a result, the slider 10 moves relative to the base 1 in the direction I during the rod 5 is vibrating.
When the slider 10 is to be moved in the arrow direction II on FIG. 1B, the vibration waveform of the piezoelectric element 4 and the rod 5 would be a reverse of the waveform shown in FIG. 2, and such the reversed waveform has alternately an acute rising portion and a gentle falling portion. The principle for moving the slider 10 is the same as described above.
As mentioned above, it is needed to vibrate the rod 5, with which the slider 10 is frictionally engaged, with drawing a sawtooth-like waveform. One of the methods for causing such the vibration is disclosed in the Japanese patent publication of JP-A-11-41953.
According to the Japanese patent publication of JP-A-11-41953, a plurality of piezoelectric elements are disposed in series. To each of the piezoelectric elements, a waving voltage expressed by a laterally symmetric pulse waveform (such as a sine wave) is applied. But, the waving voltages are of different amplitudes and cycles from each other. Thus, in a total observation during the rod's vibration, the moving speed of the rod in one direction and the moving speed in the other direction are made different from each other.
According to the Japanese patent publication of JP-A-11-41953, the rod can be vibrated drawing sawtooth-like waveforms, by applying a voltage with simple sine waveform to each of the piezoelectric elements, without the need of generating a voltage with sawtooth-like waveform. Therefore, this method has an advantage in that the constitution of the drive circuit can be simplified.
In general, the gain can be the local maximum when a resonance frequency is input to a physical vibration system. That is, in the case where the driving is caused by inputting alternating signals to a plurality of piezoelectric elements as in the Japanese patent publication of JP-A-11-41953, highly efficient driving would be realized if a resonance frequency of the vibration system is input.
Accordingly, in the case of using a plurality of piezoelectric elements, it becomes possible to realize highly efficient driving and the energy consumption can be saved by the followings: that is, a first resonance frequency is input to a first piezoelectric element, a second resonance frequency is input to a second piezoelectric element, and the same is repeated, so that an “n”th resonance frequency is input to an “n”th piezoelectric element.
If the resonance frequencies are in a relationship of integral multiple (that is, f1=f2/2= . . . =fn/n, where the first resonance frequency is f1, the second resonance frequency is f2, . . . , and the “n”th resonance frequency is fn), inputting each of the resonance frequencies to each of the piezoelectric elements respectively would realize a regular sawtooth-like vibration waving with a constant cycle.
However, in case that the resonance frequencies are not in the above-mentioned relationship of integral multiple (in general, in a certain physical vibration system, it is not always that the resonance frequencies are in the relationship of integral multiple), the local maximum value of the gain is not regularly obtained even if each of the resonance frequencies is input to each of the piezoelectric elements. Therefore, a regular sawtooth-like vibration waveform consisting of regularly repeated same wave-sections can not be obtained. In other words, highly efficient driving with taking advantage of resonance can not be realized.